In the process of perf,Broker shutdown.

### Broker logs
16:12:59.243 [BookKeeperClientWorker-OrderedExecutor-2-0] ERROR org.apache.bookkeeper.common.util.SafeRunnable - Unexpected throwable caught
java.lang.NullPointerException: null
at org.apache.bookkeeper.mledger.impl.OpAddEntry.lambda$handleAddFailure$0(OpAddEntry.java:291) ~[org.apache.pulsar-managed-ledger-2.8.1.jar:2.8.1]
at org.apache.bookkeeper.mledger.util.SafeRun$1.safeRun(SafeRun.java:32) ~[org.apache.pulsar-managed-ledger-2.8.1.jar:2.8.1]
at org.apache.bookkeeper.common.util.SafeRunnable.run(SafeRunnable.java:36) [org.apache.bookkeeper-bookkeeper-common-4.14.2.jar:4.14.2]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_181]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_181]
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) [io.netty-netty-common-4.1.66.Final.jar:4.1.66.Final]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_181]

16:12:59.344 [pulsar-io-4-27] WARN org.apache.pulsar.broker.service.BrokerService - Namespace is being unloaded, cannot add topic persistent://my-tenant/my-namespace/test-partition-1
16:12:59.345 [pulsar-io-4-27] WARN org.apache.pulsar.broker.service.ServerCnx - [/192.168.149.32:43464][persistent://my-tenant/my-namespace/test-partition-1][sub-0] Failed to create consumer: consumerId=7, Namespace is being unloaded, cannot add topic persistent://my-tenant/my-namespace/test-partition-1

16:12:59.510 [pulsar-io-4-58] WARN org.apache.pulsar.broker.service.persistent.PersistentTopic - [persistent://my-tenant/my-namespace/test-partition-5] Attempting to subscribe to a fenced topic
16:12:59.510 [pulsar-io-4-58] WARN org.apache.pulsar.broker.service.ServerCnx - [/192.168.149.32:43516][persistent://my-tenant/my-namespace/test-partition-5][sub-0] Failed to create consumer: consumerId=20, Topic is temporarily unavailable

Start with the broker log stack trace in OpAddEntry.java, especially handleAddFailure, then trace the related BrokerService, ServerCnx, and PersistentTopic messages during namespace unloading and topic fencing. Reproduce or correlate the shutdown sequence and determine the cause of the NullPointerException and failed consumer creation; done means the shutdown no longer produces these failures.

Written by the indexing model from the issue text.